Tunable filter for RF circuits

Abstract

A tunable filter is described where the frequency response as well as bandwidth and transmission loss characteristics can be dynamically altered, providing improved performance for transceiver front-end tuning applications. The rate of roll-off of the frequency response can be adjusted to improve performance when used in duplexer applications. The tunable filter topology is applicable for both transmit and receive circuits. A method is described where the filter characteristics are adjusted to account for and compensate for the frequency response of the antenna used in a communication system.

What is claimed is: 
     
       1. A tunable filter comprising:
 a first operational amplifier comprising a non-inverting input and an output thereof; and 
 a second operational amplifier comprising a non-inverting input and an output thereof, 
 the output of the first operational amplifier connected to the non-inverting input of the second operational amplifier at an output of the tunable filter; 
 the output of the second operational amplifier connected to the non-inverting input of the first operational amplifier at an input of the tunable filter; and 
 a tunable capacitor coupled in between the non-inverting input of the first operational amplifier and ground, 
 wherein the output of the second operational amplifier is connected to a variable resistor circuit, the variable resistor circuit connected to the non-inverting input of the first operational amplifier. 
 
     
     
       2. The tunable filter of  claim 1 , wherein a change in resistance of the variable resistor is operable to result in an inverse change in frequency of a filter response of the tunable filter. 
     
     
       3. The tunable filter of  claim 1 , wherein the variable resistor circuit comprises a plurality of resistors coupled to a multi-port switch. 
     
     
       4. The tunable filter of  claim 1 , wherein the variable resistor circuit is connected in a series configuration with the non-inverting input of the first operational amplifier prior to a junction where the tunable capacitor is connected between the non-inverting input of the first operation amplifier and ground. 
     
     
       5. The tunable filter of  claim 1 , further comprising a second variable resistor circuit coupled to the non-inverting input of the first operational amplifier. 
     
     
       6. A communication system comprising the tunable filter of  claim 1 . 
     
     
       7. The communication system of  claim 6 , wherein the communication system comprises an antenna. 
     
     
       8. The communication system of  claim 7 , wherein the antenna is an isolated magnetic dipole. 
     
     
       9. The communication system of  claim 7 , wherein the tunable fitter is disposed in a receive chain. 
     
     
       10. The communication system of  claim 7 , wherein the tunable filter is disposed in a transmit chain.